LONDON — Police said Thursday the 39 people found dead inside a tractor-trailer at an industrial park in southeast England “are believed to be Chinese nationals,” with eight of the victims women.

In one of the country’s biggest ever murder investigations, police carried out raids and searched three properties in Northern Ireland, where the 25-year-old driver of the refrigerated truck is from.

Britain’s National Crime Agency said it was working to identify “organized crime groups who may have played a part.”

Essex police said the driver, identified as a 25-year-old from Northern Ireland, had been arrested on suspicion of murder.

The bodies of the 39 adults were found at Waterglade Industrial Park in Grays, about 25 miles east of central London. Police have not yet offered an account of what might have happened, but the scene bore the markings of human trafficking.

The Chinese embassy in London said in a statement, “We read with heavy heart the reports about the death of 39 people in Essex, England. We are in close contact with the British police to seek clarification and confirmation of the relevant reports.”

In an update on Thursday morning, police said that the truck itself entered the country via Holyhead in North Wales on Sunday, traveling from Dublin.

Essex Police and Belgian prosecutors said the container part arrived by sea from Zeebrugge, a Belgian port, to Purfleet, a small port in Essex on the River Thames, docking shortly after 12:30 a.m. on Wednesday. 

At about 1:05 a.m. Wednesday, the truck and trailer left Purfleet, police said and 35 minutes later, police received a call from local ambulance services saying they had discovered the container. It was unclear how the ambulance services had been alerted.

The Belgian Federal Public Prosecutor’s Office announced it had opened a case to focus on who was behind the transport. The Belgian investigators said they did not know when the migrants were loaded into the shipping container.

The case echoed previous fatal incidents involving migrants smuggled in containers.

In June 2000, the bodies of 58 Chinese immigrants were found in the back of a shipping container in the English port city of Dover. The following year, a Dutch driver was sentenced to 14 years in jail for manslaughter. The immigrants, who paid a smuggling gang $26,000, suffocated after the driver closed a vent on the truck during a five-hour ferry ride across the English Channel.

“We continue to work diligently to piece together the circumstances of this horrific event, which has led to the largest murder investigation in our force’s history,” the Essex police force said in a statement. 

Police on Wednesday did not name the driver, though several British media outlets identified him, citing sources in Northern Ireland, and posted photos from what were said to be his social media accounts.

The truck was registered in Varna, Bulgaria — a port city on the Black Sea — to a company owned by an Irish citizen, according to a statement by the Bulgarian Foreign Ministry. Bulgarian Prime Minister Boyko Borissov told a local television broadcaster that the truck left immediately after it was registered in 2017 and hadn’t returned.

British Prime Minister Boris Johnson said Wednesday he was “appalled by this tragic incident in Essex.”

“I am receiving regular updates and the Home Office will work closely with Essex Police as we establish exactly what has happened. My thoughts are with all those who lost their lives & their loved ones,” he tweeted.

British authorities say human trafficking and modern-day enslavement are on the rise. National Crime Agency figures show that nearly 7,000 possible victims were reported last year — a 36 percent increase from 2017. Those people came from 130 countries, with Albanians and Vietnamese the most common foreign nationalities.

In August 2015, 71 bodies were found on a highway in Austria, inside a sealed and locked freezer truck. Most of the people were from Syria, Iran, Iraq and Afghanistan. The discovery came at the peak of Europe’s refugee influx and became one of its defining, tragic moments.

Research by German public television and the Sueddeutsche Zeitung newspaper later revealed that Hungarian officials had tapped the traffickers’ phones but failed to act in time. 

After the 2015 incident, the E.U. law enforcement agency Europol added a dedicated European Migrant Smuggling Center. In a report published this year, the center found that the most common method of smuggling involves hiding people inside vehicles.

Rod McKenzie, managing director of policy and public affairs at the Road Haulage Association, said the journey for the people in the truck found in Essex would have been “hellish.” He said it was clear from pictures that the truck had a refrigerated unit, where temperatures can go as low as minus-13 Fahrenheit.

“It would be completely dark, probably completely airless, no sanitary facilities, possibly freezing temperatures, with the likelihood of death from freezing or suffocation enormous,” McKenzie said.

He surmised that those who sent the trailer to Essex may have chosen the route in an effort to avoid the strict checks at the popular crossing between Calais, France, and Dover. He said authorities there use sniffer dogs and monitors that can detect heartbeats, heat and CO2 levels, among other things.

“Purfleet, however, doesn’t have that level of technology to screen lorries,” he said.

Rick Noack in Berlin contributed to this report.